Electron microscope tomography (EMT) is the method of computational reconstruction of the 3-D electron density of an ultrastructural specimen from combined projections around a tilt axis. The development of EMT represents a team effort of electron microscopists, a mathematician/crystallographer, a computer graphic programmer and a cell biologist. The work has and will continue to emphasize analysis of stained plastic-embedded sections tilted around a single axis, representing a logical extension of much of conventional biological transmission EM. Since the method is now maturing and has received several applications, the long-term objectives of the research has also evolved. The development of sophisticated 3-D computer graphics tools for the analysis and modeling of complex, generally asymmetric, data-intensive reconstructed objects is proposed. Performing EMT on thick sections (>0.25 mum) is an important application of the method, especially when dealing with the large-scale organization of cellular organelles. Analyses of thick sections imaged with inelastically- scattered electron and examination of mass-loss (specimen-damage) by electron spectroscopy are planned. In addition, studies of various reconstruction algorithms, especially the POCS (projections onto convex sets) approach, with regard to behavior in a limited-angle tilt series are proposed. The significance of EMT is considerable. Combined with advances in light microscopy, EMT will result in a more profound understanding of the patterns and principles of 3-D cellular organization in normal and pathologic states. Furthermore, some of the 3-D computer graphics tools under development in this proposal should be of use to the problems encountered in the analysis of 3-D medical images employing data collected by CAT,PET and MRI techniques.